Examples of lithium secondary batteries include batteries using an organic electrolyte (electrolyte in the form of a solution of an organic substance, hereinafter also referred to as an “organic electrolytic solution”) and batteries using a solid electrolyte (hereinafter also referred to as a “solid electrolyte layer”). Above all, batteries in which a lithium (Li) based metal is used for a negative electrode (hereinafter also referred to as a “negative electrode layer”) have a high discharge capacity per unit volume and thus are considered to be excellent. However, in the former, if metallic Li is used for a negative electrode, as charging and discharging are repeated, the metallic Li may react with the organic electrolytic solution to form needle crystals, resulting in a decrease in charge/discharge cycling characteristics, and furthermore the needle crystals may penetrate a separator and reach a positive electrode (hereinafter also referred to as a “positive electrode layer”), thus causing short circuiting. In order to prevent such a problem, various means have been employed, for example, use of a composite material in which carbon and a Li metal foil are laminated together, use of Wood's metal, and alloying. Furthermore, in many cases, the batteries using the organic electrolytic solution cannot withstand the temperature in the reflow solder mounting process, and thus heat resistance is insufficient. On the other hand, in the latter using the solid electrolyte, although heat resistance is sufficient in the reflow solder mounting process, means for improving the heat resistance of Li having a melting point of about 180° C. is required. Various measures have been taken for this purpose. For example, Japanese Unexamined Patent Application Publication No. 2004-179158 (Patent Reference 1) and Japanese Unexamined Patent Application Publication No. 2004-127743 (Patent Reference 2) introduce negative electrode materials, such as a composite material in which metallic Li or particles thereof are embedded in a carbon-based material.
Lithium secondary batteries having a basic construction in which a solid electrolyte layer is disposed between a positive electrode layer composed of LiCoO2 or the like and a negative electrode layer containing Li have been mainly produced by a method in which these components are deposited from a vapor phase (hereinafter also referred to as a “vapor-phase synthesis method”) or by a method in which powder molding is carried out (hereinafter also referred to as a “powder method”). Furthermore, as introduced in the above-mentioned Patent Reference 2 and Journal of Non-Crystalline Solids, 123 (1990) pp. 328-338 (Non-Patent Reference 1), examples of the material for the solid electrolyte layer include Li compounds mainly containing phosphorus (P) and sulfur (S) and compounds obtained by further incorporating oxygen (O) into these compounds. As introduced in the above Patent Reference 1, examples also include Li compounds containing niobium (Nb), tantalum (Ta), and oxygen (O).
In the case of the powder method, starting material powders have been formed mainly by a technique of quenching and solidifying a melt (melt-quenching or another quenching technique) or by a mechanical milling (MM) technique in which powders are allowed to react with each other by mixing using a ball mill or the like. These powders are glassy and/or crystalline and in the form of disks, lumps, or flakes. The former is introduced in, for example, the above-mentioned Non-Patent Reference 1 and Japanese Unexamined Patent Application Publication No. 4-231346 (Patent Reference 3), and the latter is introduced in, for example, Japanese Patent No. 3233345 (Patent Reference 4) and Japanese Unexamined Patent Application Publication No. 2004-265685 (Patent Reference 5).
Examples of the secondary batteries containing a solid electrolyte include a structure in which battery components including a positive electrode layer, a solid electrolyte layer, and a negative electrode layer, each in a thin film form, are deposited by the vapor-phase synthesis method, a structure in which the battery components are deposited on a metal thin film disposed on a ceramic base or on a current collector layer composed of a metal foil, and a structure in which powder compacts are used for a positive electrode layer and a solid electrolyte layer, and the battery components, each in a thick film form, are disposed.
Representative examples of the solid electrolyte include an electrolyte composed of LiPON obtained by nitriding of lithium phosphate and an electrolyte composed of an amorphous Li2 O—V2O5—SiO2-based oxide. Incidentally, Japanese Unexamined Patent Application Publication No. 10-83838 (Patent Reference 6) introduces the latter material with improved charge/discharge cycling characteristics. In secondary batteries in which these materials are used for solid electrolytes, in the case of a battery of a thin film type, the Li ionic conductivity is about 10−6 S/cm, the current density is about 10 μA/cm2, and the capacity is about several tens of microampere hours (μAh) at the most. Consequently, the value is considerably lower than about 3 mAh/cm2 of ordinary secondary batteries using an organic electrolytic solution, and high-speed charging is difficult. Thus, they are not of a practical level.
Under these circumstances, in order to improve the current density, attempts have been made in which sulfide-based solid electrolytes, such as Li2S—P2S5-based solid electrolytes, having high Li ionic conductivity comparable to that of the organic electrolytic solutions are used. However, the sulfide-based solid electrolytes are electrochemically unstable against a negative electrode containing metallic Li and easily decompose. Incidentally, solid electrolytes containing sulfur (S) and phosphorus (P) having relatively high lithium ionic conductivity are introduced in, for example, Japanese Unexamined Patent Application Publication No. 2003-68361 (Patent Reference 7), in which, however, carbon is used as a negative electrode. Meanwhile, Japanese Unexamined Patent Application Publication No. 4-231346 (Patent Reference 3) and Japanese Examined Patent Application Publication No. 6-54687 (Patent Reference 8) show lithium secondary batteries having a negative electrode composed of metallic Li in which a solid electrolyte material containing oxygen (O) and a halogen (Cl, I, or the like) in addition to sulfur (S) and phosphorus (P) is used. However, it is known that the potential window is relatively narrowed by the addition of LiI or the like. Furthermore, Japanese Patent No 3716833 (Patent Reference 9) discloses a battery in which a negative electrode is composed of a solid electrolyte material containing oxygen (O) in addition to sulfur (S) and phosphorus (P), which has a relatively high capacity and in which charge/discharge cycling characteristics are improved.
[Patent Reference 1] Japanese Unexamined Patent Application Publication No. 2004-179158
[Patent Reference 2] Japanese Unexamined Patent Application Publication No. 2004-127743
[Patent Reference 3] Japanese Unexamined Patent Application Publication No. 4-231346
[Patent Reference 4] Japanese Patent No. 3233345
[Patent Reference 5] Japanese Unexamined Patent Application Publication No. 2004-265685
[Patent Reference 6] Japanese Unexamined Patent Application Publication No. 10-83838
[Patent Reference 7] Japanese Unexamined Patent Application Publication No. 2003-68361
[Patent Reference 8] Japanese Examined Patent Application Publication No. 6-54687
[Patent Reference 9] Japanese Patent No. 3716833
[Non-Patent Reference 1] Journal of Non-Crystalline Solids, 123 (1990) pp. 328-338